This subclass allows you to switch souls giving you different skills, personality or even appearances.

Soul Switch

You need to make a wisdom saving throw equal to your ki save dc if you fail your dm decides what soul you become if you succeed you stay as you are.

Assassin: You gain advantage on stealth checks and dexterity saving throws as well as the rogue sneak attack feature on your unarmed strikes.

Soldier: You gain advantage on athletics checks and strength saving throws as well as the barbarian reckless attack feature.

Strategist: You gain advantage on investigation and perception checks as well as on intelligence saving throws. 

Advisor

You can spend 2 ki points as a bonus action to gain the advantages from your other souls e.g if you were the soldier you can gain the assassin's advantage on stealth checks and dexterity saving throws.

Protection of Souls

Your souls are able to protect your mind giving you advantage against being charmed or frightened

Soul Master

You are now a master of souls allowing you to switch souls for free on your action as well as allowing you to create duplicates of yourself that mimic your other souls and they have their turns directly after you.